Examples of conventional thin-type light emitting diode lamp are disclosed in Patent Document 1 and Patent Document 2. In these light emitting diode lamps, a groove-shaped recess having an elongated rectangular opening is formed at the front end surface of the lamp body. A light emitting diode chip is mounted at a substantial center of the inner bottom surface of the groove-shaped recess. The side surfaces of the groove-shaped recess are inclined to cause the light from the light emitting diode chip to be emitted from the elongated rectangular opening.
In recent years, a GaN-based light emitting diode chip which emits blue light has been developed. The blue light emitting diode chip is known to exhibit high brightness. As disclosed in Patent Document 3 and Patent Document 4, such a blue light emitting diode chip can be used for a white light emitting device designed to emit white light.
The conventional white light emitting device disclosed in these documents includes a red light conversion layer made of a light-transmitting synthetic resin containing powder of a red fluorescent material which emits red light when excited by blue light, and a green light conversion layer made of a light-transmitting synthetic resin containing powder of a green fluorescent material which emits green light when excited by blue light. With this arrangement, by causing the blue light emitted from the blue light emitting diode chip to pass through the red light conversion layer, red light is emitted from the red light conversion layer. By causing the blue light after passing through the red light conversion layer to pass through the green light conversion layer, green light is emitted from the green light conversion layer. The blue light emitted from the blue light emitting diode chip, the red light emitted from the red light conversion layer and the green light emitted from the green light conversion layer are combined to produce white light for emission.
In the structure disclosed in Patent Document 3, the green light conversion layer is formed in close contact with the obverse surface of the blue light emitting diode chip, and the red light conversion layer is formed in close contact with the outer surface of the green light conversion layer. In the structure disclosed in Patent Document 4, a red light conversion layer and a green light conversion layer formed in close contact with the outer surface of the red light conversion layer are arranged at a position separate from the blue light emitting diode chip.
By applying the above-described white light emitting device to a thin-type light emitting diode lamp having the above-described structure, a thin-type light emitting diode lamp which emits white light is obtained. Specifically, in the thin-type light emitting diode lamp, a blue light emitting diode chip is mounted at a substantial center of the inner bottom surface of the groove-shaped recess. The blue light emitting diode chip is covered with a red light conversion layer, and a green light conversion layer is formed on the outer side of the red light conversion layer. With this arrangement, white light is emitted from the elongated rectangular opening of the groove-shaped recess of the thin-type light emitting diode lamp.                Patent Document 1: JP-A-2001-36147        Patent Document 2: JP-A-2005-317820        Patent Document 3: JP-A-2002-510866        Patent Document 4: JP-A-2005-228996        
The wavelength of blue light is about 450 nm, which is the shortest. The wavelength of green light is about 530 nm. The wavelength of red light is about 650 nm, which is the longest. Light having a shorter wavelength is more likely to be absorbed than light having a longer wavelength. Thus, when a red light conversion layer and a green light conversion layer are arranged in close contact with each other, part of the green light emitted from the green light conversion layer, which has a shorter wavelength, is directly absorbed by the red light conversion layer. Thus, the amount of the green light emitted from the green light conversion layer reduces by as much as the amount absorbed by the red light conversion layer. The green light absorbed by the red light conversion layer is converted into thermal energy to be consumed.
Thus, in the above-described arrangement, to make the light obtained by conversion close to white light, the amount of green light needs to be increased. For this purpose, the content of the green fluorescent material in the green light conversion layer needs to be increased. Further, since green light is absorbed, the brightness of the white light deteriorates correspondingly.
Moreover, since the groove-shaped recess is elongated, the path of light emitted from the blue light emitting diode chip becomes longer as progressing away from the blue light emitting diode chip in the longitudinal direction of the groove-shaped recess. Thus, of the light emitted from the blue light emitting diode chip, the part to be emitted from the portions adjacent to the two ends of the elongated rectangular opening of the groove-shaped recess is not sufficiently converted into red light by the red light conversion layer, because it travels through a relatively long path. As a result, the white light emitted from different portions of the elongated rectangular opening of the groove has different tones, which is not desirable.